1. Field of the Invention
The present invention relates to a method of manufacturing a pulley with integral bearing of the type comprising a sheet metal pulley mounted on the outer race of a bearing.
2. Description of the Prior Art
Pulleys with integral bearings of the above kind are used on automobile vehicle engines, for example, to tension a drive belt of an accessory of the engine.
Prior art tensioning pulleys with integral bearings of the above kind comprise a pulley mounted on the rotating outer race of a bearing and adapted to be pressed radially against a belt. The fixed inner race of the bearing is mounted on a support and an appropriate system, for example a cam system, enables the bearing, and therefore the pulley with the integral bearing, to be moved radially relative to the belt to adjust the tension in it.
The pulley is often made from pressed sheet metal, is in one or two parts and is generally a force fit on a cylindrical bearing surface consisting of the outside surface of the outer race of the bearing.
This method of mounting the pulley on the bearing causes problems due to possible modification of the geometry of the outer race of the bearing caused by force fitting the pulley on it.
Because of the method by which it is manufactured, neither the shape nor the dimensional tolerances of a pressed sheet metal pulley has a particularly high geometrical accuracy, compared to a bearing race which has been finish ground.
Force fitting the pulley on the bearing therefore causes a number of geometrical and dimensional modifications of the bearing, which can be summarized as follows:
a) Reduction of the internal radial clearance of the bearing between the balls and the raceways. PA1 b) Possible modification of the osculation, i.e. the ratio between the radius of curvature of the grooves of the raceways in a radial section plane passing through the axis of the bearing and the diameter of the balls. PA1 c) Introduction of out-of-round defects into the outer race of the bearing, which is prejudicial to correct working of the bearing and its service life.
A relatively precise clearance, neither too large nor too small, is needed for the bearing to function correctly. PA2 It is therefore necessary to use bearings which have a greater clearance than the standard clearance in order to compensate the effects of the reduced clearance. PA2 Consideration might also be given to using bearings having a more massive outer race, of larger cross section, to limit the reduction of clearance due to force fitting, but this solution would be relatively costly because of the use of non-standard races. PA2 With the radial clearance, the osculation is one of the parameters influencing the possibility of skewing of the bearing, i.e. possible tilting of the geometrical axis of the outer race relative to the geometrical axis of the inner race. This parameter must remain within a predetermined range of values if the tension pulley with integral bearing is to work correctly.
Thus the various solutions considered above to the problem of eliminating or at least reducing geometrical and dimensional modifications caused by the usual procedure of force fitting a sheet metal pulley onto the outer race of the bearing are not entirely satisfactory.
It has also been proposed to glue a pulley to the outer race of a bearing, in particular a pulley molded from phenolic resin. However, to obtain a sufficiently strong bond between the pulley and the race, the pulley is heated and therefore expanded before it is fitted, and therefore shrinks when it cools, after it is fitted. Thus, even if very close tolerances are adhered to for the pulley and the outer bearing race, which necessarily increases the cost of the tension pulley with integral bearing, there remains the risk either of too great a clearance for a sufficiently strong adhesive bond to be obtained between the pulley and the race or of a clamping effect having the drawbacks already referred to in connection with force fitting the pulley onto the outer race of the bearing.
In an attempt to overcome these problems of fixing by gluing, WO-A-98/34053 proposes to fix a pulley, in particular a pulley molded from phenolic resin, to the outer race of a bearing by means of a molded bond obtained by injecting a molten material, for example a metal such as zinc, zinc alloy or lead or a plastics material such as polyamide, between the outer race of the bearing and the pulley which surrounds that race with an interstice or clearance of at least 0.5 mm between the two parts. This involves the use of means for centering the bearing relative to the pulley, special injection equipment and most importantly a specific injection mold for each size of bearing and pulley. What is more, the molded bond established in this way implies a particular conformation of the pulley and the outer race of the bearing, which rules out the use of standard bearings and pressed sheet metal pulleys.
This bonding method therefore cannot be used to manufacture pulleys with integral bearings at low cost.
The object of the present invention is to provide a method of manufacturing pulleys with integral bearings simply and at low cost and which remedies the drawbacks and problems of the prior art methods.